Characterization of the bottom ash in municipal solid waste incinerator
Introduction
Municipal solid waste incineration (MSWI) reduces the volume of waste by about 90% 1, 2, and its mass by about 70%. MSWI residues (bottom ash, grate sifting, heat recovery ash, fly ash and air pollution control (APC)), are generated at different points in the process of MSWI. Approximately 80% of incineration residue is bottom ash. At present, about 17 Mt/yr [3]of bottom ash in the world is produced. This is expected to double in the next ten or fifteen years. Normally, the term `bottom ash' also includes grate siftings and, depending on the facility design, heat recovery ash.
The incineration process is not the final waste treatment stage. Municipal waste reutilization and disposal options are the focus of debate. In the USA, most MSWI residue currently generated is landfilled, while in some European countries (e.g. Germany, the Netherlands, France and Denmark) about 50% of the stockpiled municipal waste incinerator bottom ash is used as secondary building material, in road construction or as raw material for the ceramic industry inter al. 2, 3, 4, 5, 6, 7.
Chemical analyses of MSWI residue, bottom ash, APC residue and combined ash, have often been published [3]. The fact that compositions differ very slightly indicates a relative stability in the proportions of materials in bottom ash, independently of their origin and the particular incineration process. Many leaching tests have been designed by regulatory agencies to characterize trace element mobility and to simulate a field leaching scenario with which the amounts of toxic trace elements available for leaching can be estimated [8].
Bottom ash, which under the regulations of some European countries can be reused as secondary building material or similar, generally contains low concentrations of heavy metals, especially volatile species such as lead, cadmium and zinc. These small amounts of heavy metals are mainly concentrated in the heat recovery ashes and the grate sifting materials, which are collected together with bottom ashes [3], and consist of particle size fractions under 1 mm. However, little research has been conducted on the characterization of the main materials present in bottom ash in particles bigger than 1 mm. The chemical analysis of this particle size fraction gives poor information, which means that a proper evaluation of this materials' reuse and recycling potential cannot be made.
Eighmy et al. [9]described a particle classification from bottom ash but did not take into account this classification for each size distribution of the municipal waste incinerator. This paper reports the most significant results of the characterization of the main materials in the bottom ash particles with diameters between 1 and 25 mm coming from two types of MSWI facilities. This size fraction represents more than 85% of bottom ash [2]and about 68% of the total amount of the solid residue streams (bottom ash and APC residue) produced by a MSWI facility. This characterization can be used to evaluate these materials final disposal, eventual utilization or possible recycling. The amount of glass and other material remaining in bottom ash could be a parameter for evaluating the effectiveness of the municipal voluntary recycling programs.
Section snippets
Methods and materials
The bottom ash used in this study came from two MSWI facilities in Catalonia (Spain) which used energy recovery (waste-to-energy, WTE).
Facility A is located in the metropolitan area of Barcelona and began to operate in 1975. In 1996, it handled 302 900 tons (≅910 tons/day) of mainly household waste stream with some commercial contributors and produced electric power (118 900 MW h), scrap iron (9000 tons), 77 400 tons of bottom ash and 2500 tons of fly ash collected by electrostatic filters. At
Particle size distribution
It is surprising to observe that, in all samples of bottom ash from A and B facilities, particles in the 25–250 mm size range were practically non-existent. The trommeled particles over 250 mm were mainly from building or domestic metallic articles. Bottom ash residue comprises mainly ceramic materials, silicates, phosphates, sulfates or carbonates, which are easily broken down by the mechanical system of transport within the furnace or by the effect of the thermal shock. It is possible to find
Acknowledgements
We would like to thank Mr F. Gatuellas and Mr Ramon Nadal, General Managers of TERSA and SIRUSA , respectively, for their cooperation in financing and supporting this investigation, the Serveis Cientı́fico-Tècnics of the University of Barcelona and the Fundació Bosch i Gimpera. We also would like to thank Mr Robin Rycroft for linguistic and stylistic advice.
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